1. Field of the Invention
The present invention relates to a partial discharge monitoring method and system to diagnose failure condition of a gas-insulated apparatus by sensing partial discharge caused inside the gas-insulated apparatus.
2. Prior Art
As power supply increases and substation facilities become more compact recently, gas-insulated apparatus filled with high-pressure SF6 gas of which electrical insulation performance, current limiting performance and reliability are very excellent has been popularized.
While SF6 gas filled in a gas-insulated apparatus exhibits excellent insulation characteristic, the insulation characteristic degrades drastically in a non-uniform field. This is mostly caused by internal failures, particularly mixture of foreign substance in the apparatus, protrusion formed on internal conductor, and void in insulated spacer. If any of this internal failure exists, partial discharge is caused and may progress when applied with high voltage or when subjected to overvoltage such as lightning, possibly resulting in dielectric breakdown in the end.
Accordingly, one of effective means for preventing dielectric breakdown accident is to sense partial discharge, premonitory symptom of dielectric breakdown, sensitively and several sensing methods based on electromagnetic wave, sound, light or producted gas have been proposed. In particular, since a method that employs electromagnetic wave signals in the frequency band from several-hundred MHz to several GHz is hard to be affected by noise, vital research and development has been made on it as a method for sensitive partial discharge sensing.
These sensors for electromagnetic wave, installed in a gas-insulated apparatus as disclosed in the Patent Document 1, for example, realizes sensitive sensing of electromagnetic wave due to partial discharge. In this method using electromagnetic wave, the frequency of the signal sensed by the sensor is analyzed in a frequency band with less noise, from several-hundred MHz to several GHz, and existence of partial discharge signal is checked. In addition to the method using frequency analysis, as disclosed in the Patent Document 2, there has been proposed a method for determining the occurrence of partial discharge from the phase pattern that is synchronized with the phase of commercial frequency voltage being applied. Furthermore, another method employing an estimation technique such as neural network to determining defect type from the sensed signal has also been proposed as disclosed in the Patent Document 3.
There are some other methods for specifying the location of occurrence of partial discharge; including a method for determining the location from the difference of time in reaching two sensors and, as disclosed in the Patent Document 4, a method for estimating the location of partial discharge inside a gas-insulated apparatus by utilizing the attenuation characteristic of electromagnetic wave signal.
There has also been proposed another method, as disclosed in the Patent Document 5, with which a signal resulting from insulation failure, energization failure or mixture of fine foreign substance is sensed and the failure is diagnosed collectively in an expert system by comparing the signal with stored past signal data and the remedial action guidance and life expectation based on the result are displayed. To be concrete, the patent describes that the length of defect can be estimated from the correlation between the voltage applied to the gas-insulated apparatus and discharge level. According to studies later on, however, it has been found that the characteristic of applied voltage vs. discharge level does not correspond to the defect size in most cases.
[Patent Document 1] Patent Publication No. 3299547
[Patent Document 2] Japanese Application Patent Laid-open Publication No. HEI 10-170596 (1998)
[Patent Document 3] Japanese Application Patent Laid-open Publication No. 2000-224723
[Patent Document 4] Japanese Patent Publication No. HEI 7-50147 (1995)
[Patent Document 5] Japanese Application Patent Laid-open Publication No. HEI 7-49362 (1995)